How high sea levels may rise has remained a subject of considerable debate among the climate cognoscenti. Even as their models have grown ever more sophisticated, researchers have struggled to make sense of some of the underlying physical mechanisms driving sea level fluctuations—particularly the response of the Greenland and Antarctic ice sheets to global warming. This, on top of the intricacies inherent to a complex system, has left sea level predictions subject to significant uncertainties.

The most recent report of the Intergovernmental Panel on Climate Change (IPCC), published in 2007, estimated that sea levels would rise by 18 to 59 cm by 2100. It couched its forecast in caveats, however, acknowledging that even the most advanced models did not consider the full range of changes in ice sheet flow and uncertainties in climate-carbon cycle feedbacks. Devising a single model that accurately incorporates the full array of parameters and processes that shape the global climate system has been a continual challenge for researchers, leaving most models focused on a particular variable, such as temperature, around which they center their prognostications.

To tackle changes in sea levels, a team of researchers led by Mark Siddall of Columbia University opted for a different approach, looking to the past by drawing on reconstructions of sea-level rise, derived from fossil corals, which span the past 22,000 years—since the Last Glacial Maximum (LGM). They used this to create a simpler, integrated model of sea-level response, which they detailed in the latest issue of Nature Geoscience.

The advantages of this approach are two-fold. First, because the model covers such a lengthy period, the authors were confident that it at least implicitly includes the contributions from thermal expansion, which occurs when water expands as it heats up, and the changes in glaciers and ice sheet flows. Second, and, perhaps, more importantly, it enabled them to carefully scrutinize the non-linear response of sea levels to large temperature fluctuations on centennial timescales— most current models are only accurate on shorter time scales.

Because temperatures in the Northern and Southern hemispheres do not change in synchrony, Siddall and his colleagues considered two temperature proxy records to track the impacts of temperature variations on sea level: the oxygen isotope record of the North Greenland Ice Core Project (NGRIP), which is representative of the Northern Hemisphere, and the deuterium record of the European Project for Ice Coring in Antarctica (EPICA) Dome C, which is representative of the Southern Hemisphere.

After inputting the data and letting the models run, they found that the NGRIP data tracked the structure of the sea-level record very closely during the different phases of the deglaciation, while the EPICA Dome C data do not, failing even to track the transition to the Holocene, the current interglacial period, which began around 10,000 years ago. The NGRIP scenario estimated 4 to 24 cm of sea-level rise during the twentieth century, in good agreement with the IPCC’s report, and 2 to 6 cm per century before the industrial period. However, it was unable to resolve sea level variations on a decadal timescale.

Projecting their model ahead to 2100, they concluded that sea levels could rise by as much as 82 cm and as little as 7 cm, depending on the warming estimates drawn up by the IPCC (6.4°C and 1.1°C, respectively). When one includes the additional 9 to 17 cm rise that the IPCC estimates will come from accelerated ice-sheet flow over the next century—this raises the upper bound of sea-level rise from 59 cm to 76 cm—their numbers compare favorably to the IPCC’s.

Looking even further ahead, their model predicts that the impact of twentieth century warming will continue to drive sea level rises over the coming centuries. In that sense, the IPCC’s focus on establishing a value for the end of the current century doesn’t provide a complete picture of the challenges.

Tuesday, July 28, 2009

Reputed scientific journal, Nature GeoScience is this month running a special focus on sea level.

In their report published in 2007, the Intergovernmental Panel on Climate Change projected that sea level is likely to rise between 18 and 59 centimetres by 2100, threatening the homes and livelihoods of millions who live in low-lying and deltaic regions. This focus draws together studies of past and present sea-level change, and predictions for future fluctuations, as well as presenting insights into the challenges facing coastal communities.

Follow the link to download commentaries, feature papers and research letters on the issue of sea level rise.

PARIS — The UN's climate panel has been backed over a key question as to how far global warming will drive up sea levels this century, a study published on Sunday says.

The UN experts are right that the oceans are unlikely to rise by an order of metres (many feet) by 2100, as some scientists have feared, it says.

But, its authors caution, low-lying countries and delta areas could still face potentially catastrophic flooding if the upper range of the new estimate proves right.

In a landmark report in 2007, the UN's Intergovernmental Panel on Climate Change (IPCC) predicted oceans would rise by 18-59 centimetres (7.2 and 23.6 inches) by 2100.

The increase would depend on warming, estimated at between 1.1 and 6.4 degrees Celsius (1.98-11.52 degrees Fahrenheit) this century, which in turn depends on how much man-made greenhouse gas is poured into the atmosphere.

It based the calculation on thermal expansion of the seas -- when a liquid is warmed, it grows in volume.

Harder to calculate, the IPCC admitted, was how far meltwater from glaciers and icesheets on land would boost sea levels.

It ventured a provisional calculation, suggesting contributions from those sources could push the upper limit to 76 cms (30.4 inches).

The new paper, led by Mark Siddall of Britain's University of Bristol, used data from fossilised coral and from ice-core measurements to reconstruct sea-level fluctuations over the past 22,000 years, from the height of the last Ice Age to the balmy era of today.

This century, they calculate, the seas will rise by between seven and 82 cms, all sources included, on the basis of a 1.1-6.4 C (1.98-11.52 F) warming -- an estimated increase that is in the same ballpark as the IPCC's.

The study appears in the journal Nature Geoscience.

"Given that the two approaches are entirely independent of each other, this result strengthens the confidence with which one may interpret the IPCC results," said Siddall.

But, he said, no-one should be kidded into thinking the flooding threat was over.

"The fact that this number is smaller than other numbers does not mean that this is not potentially a massive and very important sea level rise," Siddall told AFP in a telephone interview.

"Fifty centimetres (20 inches) of rise would be very, very dangerous for Bangladesh, it would be very dangerous for all low-lying areas. And not only that, the 50 centimetres (20 inches) is the global mean. Locally, it could be as high as a metre (3.25 feet), perhaps even higher, because water is pushed into different places by the effect of gravity."

He added: "Extreme flood effects will definitely become more frequent. If you rise by 50 centimetres (20 inches), floods that once happened every 100 years then become once a decade."

Siddall also pointed out that sea levels would inevitably rise even higher after the 21st century because of inertial effect.

It takes decades for atmospheric warming to translate into a warming of the seas because of the vast volume of the ocean, he said.

Thus the 22nd century and beyond will feel the impacts of the warming of the 21st century.

The IPCC's estimates on sea levels have been repeatedly challenged since the Fourth Assessment Report was published in 2007.

Several studies have suggested that runoff from the Greenland and Antarctic icesheets -- which hold the world's biggest stores of freshwater -- will be much higher than the panel suspected.

One paper, published in April by Paul Blanchon, a geoscientist at Mexico's National University, said that, in the distant past, the seas suddenly rose by three metres (10 feet) within a very time.

There was "a distinct possibility" that a step change of this kind could happen within the next 100 years, said Blanchon.

Monday, July 27, 2009

MANILA, Philippines - Cagayan is the most vulnerable if the country’s sea level rises owing to climate change.

Cagayan, one of the country’s biggest provinces, forms part of the northeastern flank of Luzon whiplashed by the waters of the Pacific Ocean.

If the Philippine sea level rises by one meter when much of the world’s ice caps melt owing to global warming or climate change, 13,134 hectares of seashore lands in 16 Cagayan towns will go under water, states an analysis made by the University of the Philippines Los Baños (UPLB) Climate Change Program.

UPLB presented the results of its analysis at an exhibit in the just-concluded observance of National Science and Technology Week (NSTW) 2009 in Los Baños sponsored by the Los Baños Science Community (LBSC) held at the historic UPLB Baker Hall.

Palawan ranks second to Cagayan as most vulnerable, with 9,146 ha in 17 towns to submerge following a one-meter sea level rise.

Third is Iloilo with 8,647 ha in 16 towns; followed by Zamboanga Sibugay, 8,330 ha in 12 towns; and Camarines Sur, 8,139 ha in 27 towns.

A total of 264,447 ha in 180 towns will be submerged in case the country’s sea level rises by two meters owing to climate change.

In the face of such an eventuality, a comprehensive land use and development plan are needed for the provinces most vulnerable to sea level rise, the UPLB Climate Change Program stressed. – Rudy A. Fernandez

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“Without a connection to current disasters, global warming is the kind of problem people, and democratic institutions, have proved singularly terrible at solving: a long-term threat that can only be limited by acting promptly, before the harm is clear.” The New York Times, April 23, 2006